Coherent beam combiners are useful in many applications, including optical communications and laser radar. An overall receiver system, of which the beam combiner 100 is a part, is shown in FIG. 1. Radiation from an object space 102 typically enters through a mechanical scanner (not shown) before interacting the combiner itself, which merges the received wavefront with the signal from a local oscillator 104. In the balanced detection scheme, two detectors D1, D2 are used as shown, for better signal to noise ratio, for any given line of sight in the wide field of view. Using current technology, coherent beam combining is limited to the coherent field of view (FOV), given by 1.63λ/D, where λ is the wavelength of the radiation, and D is the aperture diameter. For example, for D=1 m, λ=1 micron, the coherent FOV=1.63λ/D=1 microradian=93 millionths of a degree.
In a typical system, a telescope is used to reduce the beam diameter at the input to the combiner, so that more economical optical elements with smaller small practical dimensions can be used. For example, with a beam combiner aperture size of 10 cm, a telescope with magnification M=20 has input aperture of 2 meters, and if M=100 the input aperture is 10 meters. The large telescope aperture significantly increases the light gathering capacity of the receiver.
Thus, in contrast to the coherent FOV, the total FOV is usually much larger, determined by the optical design of the receive optics (such as the telescope or lens assembly), and can be as large as 1 degree for reflecting telescopes, and several tens of degrees for wide angle camera lenses. The pointing range of receive direction can further be increased by using a mechanical scanning arrangement such as a scan mirror or gimbaled optic.
That is, while the total FOV of the system allows receiving radiation over large angles, coherent combining is limited to a very small neighborhood (=coherent FOV) of that particular direction within the total FOV. The point to note is that the coherent FOV (1.63λ/D) is extremely small compared to total FOV, unless the aperture is made extremely small, such as that of a fiber. As the aperture is made small, the collection area decreases as its square, and so the signal strength drops off dramatically. Even if fibers are used, the collection area is so small that one has to use a focusing optics before it, and the coherent FOV of the front end is limited again by the receive aperture size of the front end, and so we are back to the same problem of very small coherent FOV, even when a fiber is used at the focal plane.
The current state-of-the-art technology, as shown in FIGS. 2A and 2B, limits the field of view (FOV) of beam combiner to small values (a few μrad) in the object space. For example, for the beam combiner shown in FIG. 2A, the FOV in the object space is about (2*0.83λ/D)/M, where M is the magnification of the telescope preceding the beam combiner, λ is the wavelength of the coherent light, D is the aperture of the focusing lens. For typical values of M=20, λ=1.5 μm, and D=0.1 m, we have FOV=(1.63λ/D)/M=1.2 μrad. Device 200 is a beam splitter, and device 202 is a lens or mirror.
For a beam combiner using fiber optics (see FIG. 2(b)), the FOV in object space is approximately (d/f−λ/D)/M, where M is the magnification of telescope preceding the beam combiner, d is the core diameter of fiber, f is the focal length and D is the aperture of the lens focusing into the fiber and λ is the wavelength. For typical values of M=20, d=5 um (single mode), f=0.1 m, λ=1.5 μm, and D=0.1 m, we have FOV=(d/f−λ/D)/M=1.75 μrad.
In summary, for a given FOV, telescope magnification reduces the FOV in the object space. For example, if the Beam Combiner FOV is 20 μrad, then the FOV in the object space is only 1 μrad for M=20, and only 0.2 μrad for M=100. Accordingly, beam combiners exhibiting a wider FOV are of great value.